For the 245 tailgate struts, the official Volvo spec, as far as we know, calls for a lifting force of 400 Newton (about 90 pounds of lifting force) per strut, total for both: 800 Newton (about 180 pounds).
We did a little test and found that 700 Newton (about 157 pounds) is enough to keep a 245 tailgate up very sturdily, so even with 350 Newton (about 78 pounds of lifting force) a used one would still be fine. We plan to do some more testing to find the exact tipping point at which the tailgate barely doesn’t stay up any more. That way we can make sure we use a logical cut-off point as to the quality of the tested, guaranteed used tailgate struts we offer.
This test was performed on a 1991 car, which also has the third brake-light, automatic lock … pretty much everything we can imagine a stock 245 having.